This invention relates to a process for preparing N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid using a homogeneous catalyst system. More particularly, this invention relates to a process for producing N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid using a salt of a selected metal in the presence of a dipyridyl compound.
N-Phosphonomethylglycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. N-Phosphonomethylglycine and its salts are conveniently applied in an aqueous formulation as a postemergent phytotoxicant for the control of numerous plant species. N-Phosphononmethylglycine and its salts are characterized by broad spectrum activity, i.e., the controlled growth of a wide variety of plants.
Numerous methods are known in the art for the oxidation of the N-phosphonomethyliminodiacetic acid to N-phosphonomethylglycine. For example, U.S. Pat. No. 3,969,398 to Hershman discloses a process for the production of N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid utilizing a molecular oxygen-containing gas as the oxidant in the presence of a catalyst consisting essentially of activated carbon. U.S. Pat. No. 3,950,402 discloses a method where N-phosphonomethyliminodiacetic acid is oxidized to N-phosphonomethylglycine in an aqueous media using a free oxygen-containing gas and a noble metal catalyst, such as palladium, platinum or rhodium on a support. U.S. Pat. No. 3,954,848 discloses the oxidation of N-phosphonomethyliminodiacetic acid with hydrogen peroxide and a acid such as sulfuric acid. Hungarian Patent Application No. 011706 discloses the oxidation of N-phosphonomethyliminodiacetic acid with peroxide in the presence of metals or metal compounds.
The dipyridyl compounds useful in the process of the present invention are known to those skilled in the art to be useful as electron transfer agents. Examples of references for the latter use include: Endo, et al., Tetrahedron Letters 26, No. 37, pp 4525-4526 (1985); Ledwith, Accounts of Chemical Research, 5, pp 133-139 (1972); Farrington, et al., Chemical Communications, pp 259-260 (1969); and Rieger, et al., J. Org. Chem., 53, No. 7, pp 1481-1485 (1988).
Although satisfactory results are obtained by the processes of the prior art to prepare N-phosphonomethylglycine using heterogeneous catalysts such as activated carbon or a noble metal on a support, there is now provided a process for preparing N-phosphonomethylglycine using a homogeneous catalyst system which produces outstanding results through high conversions and selectivities, while minimizing the formation of undesirable byproducts such as phosphate. The process also simplifies separation of the product from the catalyst.